Conventional coatings used in protective layer over the sensing electrode surface of a ceramic anion-conductor exhaust gas sensor are ordinarily composed of various non-catalyzed porous ceramic oxides, such as spinel. The primary functions of these coatings are to act both as a mechanical shield to prevent gas and particulate-induced erosion of a platinum electrode of the sensor and as a filter to reduce the rate at which poisoning from silica, lead and other harmful constituents in the exhaust stream can occur. It has been observed in the past, however, that porous non-catalytic coatings have contributed to non-ideal sensor performance. In particular, the coatings are known to accentuate "lean shift", a phenomenon in which unreacted gases resulting from incomplete combustion, present in the exhaust gas, causes the sensor to switch at an air/fuel ratio which is greater than the true stoichiometric point where lambda equals exactly unity. This apparent lean shift of the sensor's switch point is caused by the faster diffusion of hydrogen as compared to oxygen through the porous protective coating covering the outer or exhaust gas electrode. In addition to "lean shift", unreacted gases can also contribute to sensor output amplitude variation and they can amplify temperature sensitivity. Although some types of coatings and sensor packaging arrangements in the marketplace seem to minimize non-ideal sensor operating characteristics better than others, these arrangements cause compromises in other aspects of the sensor's performance characteristics. No satisfactory solution to the problem has been discovered heretofore.
The present invention overcomes many of the shortcomings of the prior art.